1. Field of the Invention
The present invention relates to a thermally activated trigger device. More particularly it relates to such a device for shutting down a nuclear reactor. Still more particularly it relates to a thermally activated trigger device for rendering a space nuclear reactor subcritical upon reentry to the earth's atmosphere.
2. Description of the Prior Art
Temperature-responsive or thermally activated devices are used for a variety of control and safety cutoff functions involving many specialized applications. In several such devices a change in temperature triggers the movement of a piston in a cylinder to accomplish the desired result. Thus U.S. Pat. No. 3,638,733 shows a heat operated fire extinguisher in which a piston closing the mouth of an extinguishing agent pressure vessel is held in a sealing position by a plug of heat fuseable material. When the ambient temperature indicates the presence of a fire, the plug melts releasing the piston and discharging the pressurized vessel.
U.S. Pat. No. 4,498,491 shows a thermo-electric valve for scuttling buoyant devices. In this valve a low power resistor holds a spring in compression. Passage of a low control circuit current through the resistor disintegrates it and allows the energy of the spring to expand against the piston, which by its movement permits a pressurized medium to become active. Similarly, in U.S. Pat. No. 4,164,953 a flood valve is shown in which a piston assembly is activated by an electrical heating element surrounding a thin-walled section which is melted to permit pressure of a contained fluid to displace the piston assembly.
U.S. Pat. No. 4,422,503 shows a blow-out preventer for use in a control conduit for a downhole safety valve. A fusible link is used to hold a ball check valve in an open position. Melting of the fusible link upon the occurrence of abnormal temperature permits movement of the piston against the ball check valve to close the conduit and prevent fluid flow from the safety valve upwardly through the conduit.
U.S. Pat. No. 4,263,839 shows a heat sensitive locking device. By melting of a fusible link, a spring wire is permitted to uncoil thereby permitting a piston to release locking segments for movement to their unlocked positions.
U.S. Pat. No. 3,313,312 shows a thermally responsive actuator device in which a piston is held in place by a mass of solidified fusible material. When this fusible mass is melted and become liquid in response to an ambient temperature rise, the piston means assembly then becomes operative.
Many control and safety devices have been proposed for use with nuclear reactors, whether of the gas-cooled or liquid-cooled types, including sodium reactors. Basically, in the event of a loss of coolant or other type accident, these control and safety devices act to render the reactor subcritical (incapable of sustaining a nuclear fission reaction) and also to prevent loss of radioactive material to the atmosphere. These devices generally rely on injection of a nuclear poison into the core containing the radioactive fissionable material. Such poisons consist of any material of high absorption cross-section that absorbs neutrons unproductively and hence removes them from the fission chain reaction in a reactor, decreasing its radioactivity. Representative nuclear poisons include boron, cadmium, and hafnium. These are conveniently incorporated into control safety rods, which are then generally inserted from above into the reactor core.
U.S. Pat. No. 3,162,578 shows such a control rod having high neutron capture capabilities for use in a nuclear reactor and operating means to move the control rod at high speed when required. U.S. Pat. No. 3,432,387 shows a pressure-actuated control device for moving neutron-absorbing means into the reactor core. U.S. Pat. No. 3,855,060 shows a bottom actuated reactor control rod device. U.S. Pat. No. 3,933,581 shows another control rod drive for reactor shut-down utilizing a latching assembly. U.S. Pat. No. 4,076,584 shows a magnetically activated rodded shut-down system for a nuclear reactor.
Spacecraft, whether manned or unmanned, require electric power for several purposes. Particularly for long space voyages requiring large power supplies, chemical forms of energy and solar power have many significant limitations. In situations where large amounts of power are needed over long periods of time, the best source of electricity is a nuclear reactor. Thus, the first SNAP (Systems for Nuclear Auxiliary Power) reactor power plant was launched into space and placed in orbit in 1965. It is necessary that nuclear space reactors be rendered subcritical after reentry into the earth's atmosphere. As indicated, many devices are available for rendering terrestrial nuclear reactors subcritical, such as by inserting poison rods, inserting poison sleeves, or moving reactor segments. Such devices rely on some type of mechanical motion or electrical signal to initiate the desired actuation. Electrical signals frequently tend to be unreliable and could be activated prematurely, shutting a reactor down when not desired, or else fail to operate when required.
Accordingly, it is an object of this invention to provide a simple, passive thermally activated trigger device, free from the limitations of known trigger devices, for providing desired control or safety cutoff.
More specifically, it is another object to provide such a thermally activated trigger device particularly suitable for rendering a nuclear reactor subcritical.
It is still a further specific object of the invention to provide such a device that is effective for rendering a nuclear space reactor subcritical after reentry into the earth's atmosphere.